karma: stop excluding polyfill.js from istanbul (#30740)

js/src/dom/polyfill.js

@@ -1,5 +1,3 @@
-/* istanbul ignore file */
-
 /**
  * --------------------------------------------------------------------------
  * Bootstrap (v5.0.0-alpha2): dom/polyfill.js

js/tests/karma.conf.js

@@ -134,17 +134,17 @@ if (browserStack) {
       emitWarning: false,
       global: {
         statements: 90,
-        branches: 90,
+        branches: 89,
         functions: 90,
         lines: 90
       },
       each: {
         overrides: {
           'js/src/dom/polyfill.js': {
-            statements: 39,
-            lines: 37,
-            branches: 19,
-            functions: 50
+            statements: 30,
+            lines: 30,
+            branches: 8,
+            functions: 30
           }
         }
       }